At present, the mechanism of zona escape in vivo is poorly understood, as are the factors contributing to in vitro "hatching" from the zona pellucida. The overall goal of the present study is to obtain an understanding of the contributing factors of in vivo zona escape by studying the presence of enzyme activity in flushates that are coincident with the zona escape time window (Gonzales and Bavister, 1995) in both pregnant and pseudopregnant hamsters. Collections of embryos and flushates from the uterus during the zona escape time window will be used to compare the morphology of eggs/embryos, their location in the reproductive tract, and the presence of enzymatic activity in pregnant, pseudopregnant, and non- pregnant uteri of hamsters. Therefore, the objectives of this proposed study are: 1) to determine if there is a uterine enzyme activity that is potentially responsible for zona lysis in vivo that is coincident with the zona escape time window in both pregnant and pseudopregnant hamsters, 2) to test the hypothesis that there is no zona lytic activity in non-pregnant cycling hamster females and 3) to determine if zona lytic activity is different in vitro than the zona lytic activity in vivo. In this study the contribution of each participants involved in zona escape (the zona pellucida, the embryo and the uterine epithelium) is examined to achieve a more complete understanding of the mechanism of zona loss in utero. This study will also provide a foundation for further research to elucidate 1) regulatory effect of uterine enzyme activity associated with the critical preimplantation period, and 2) possible embryo-maternal dialogue.